This invention relates to a method for producing gaskets, seals or transition fittings which are integral to the part to which they are applied. This invention also relates to elastomer compositions, and more particularly to single or multi-component elastomer compositions and their use in forming gaskets, seals or transition fittings.
In many kinds of equipment, ducting, piping, castings, and other parts are joined together in gasketed joints in order to form gas, liquid, electrical, chemical, or sound isolation seals at the joints. The common method of making gasketed joints involves separately forming a gasket, positioning it, usually by hand, on the flange or other junction surface of one or more of the parts, attaching the gasket to one or the other of the parts by adhesive or mechanical fasteners and then bolting or otherwise fastening the two parts forming the joint together. These procedures are labor intensive and require multiple handling of parts, together with multiple needs for quality and operability inspections. Typically handling and inspections are required through three tiers of manufacturing: during production of the materials, forming of the parts, and fabrication of the assembly structure.
There is a need to be able to form or mold a gasket in place on one (or more) of the parts that is to be connected at the gasketed joint and also to form it more or less integrally with one of the parts if desired. The present invention meets that need, both for foamed gaskets and for solid or hard gaskets. It is particularly useful in the construction of molded in place gaskets for polypropylene parts.
In accordance with a principal aspect of the present invention, a method for molding the gasket, seal or transition part directly on a part is disclosed. As best illustrated by the figures described more fully below, it can be seen that a part having a bonding surface or flange is mated with a mold which is then clamped around the part, thus creating a sealed mold cavity that is generally toroidal in shape, and in which the bonding surface or flange of the part forms one or more walls of the mold. Gasket or sealant material containing a bonding agent or being otherwise rendered capable of bonding to the bonding surface is then injected into the mold cavity in molten or flowable condition. Upon solidifying, it bonds directly to the part. This eliminates the need for a separate step in which a bead of adhesive is applied to the bonding surface or flange, and then an already fabricated gasket is positioned on the surface, which is the common prior art procedure. By carefully selecting the bonding agent which is placed in the gasket or seal material, it is possible to assure that upon opening the mold, the gasket material will remain adhered to the bonding surface of the part and release cleanly from the mold. Additionally, the mold may be treated with a suitable release agent, such as a silicone release agent which is applied to the surfaces of the mold but not to the bonding surface.
Yet a further aspect of the present invention includes the application of a slip coat material to the mold which may, for example, be comprised of a water-based polyurethane. The purpose of the slip coat is to form a skin on the outer or non-bonding surfaces of the gasket. This is particularly useful in that it creates a non-tackified surface on the exterior of the gasket and effectively kills the activity of the bonding agent at these surfaces. This feature is particularly important in gasket applications where surfaces of parts made of the same materials may be joined at the gasket, but it is desirable to be able to separate them later without damaging the parts or the gasket.
In accordance another important aspect of the present invention, there is provided a new and improved elastomer composition which may be subjected to a foamed extrusion procedure to produce an elastomeric cellular product preferably in the course of which a gasket is formed. The elastomer composition of the present invention comprises a mixture of at least one thermoplastic rubber compound and a bonding agent. The at least one thermoplastic rubber compound may further comprise a mixture of two thermoplastic rubber compounds as disclosed in U.S. Pat. No. 4,677,133 issued to Leicht and U.S. Pat. No. 4,764,535 issued to Leicht, the disclosures of which are incorporated herein by reference in their entirety for all purposes.
The bonding agent may comprise one or more of any number of polymeric adhesives which are suitable for bonding the gasket or seal to a part which has already been fabricated and cooled. The bonding agent will typically comprise less than 30% by weight (5% wt. is preferable) of the mixture used to create the gasket or seal. The bonding agents which may be incorporated into the gasket material may best be illustrated by, but not limited to, several examples. If the part to which the gasket is to be bonded is formed of nylon, an acrylic bonding agent may be used. If the part to be bonded to is formed of polyethylene, it is preferred to use a hydrocarbon resin as a bonding agent. If the part to be bonded to is formed of polypropylene, then the bonding agent is preferably formed of a blend of isotactic polypropylene, atactic polypropylene and low molecular weight polyamide. Specifically, a polypropylene bonding agent may comprise approximately 5-50% fatty acid dimer based polyamide, commonly used as a curing agent for epoxy, with the balance comprising isotactic and atactic polypropylenes in a ratio ranging from 1:1 to 1:10 with a ratio of 1:5.5 being preferable.
In further accordance with the invention, a method for forming a gasket affixed to one of two parts to be joined in a gasketed joint by use of the gasket is provided. The method includes providing a mold having a walled mold cavity of the shape desired for the gasket, at least one wall of the mold cavity being a junction surface of the one part and the remaining walls of the mold cavity being formed in a mold piece. A release agent may be applied to the walls of the mold cavity except for the junction surface. The mold is closed by placing the mold piece over the junction surface of the one part to establish the mold cavity as a closed generally toroidal volume into which is injected a molten gasket-forming thermoplastic material to form the gasket. The thermoplastic material includes a bonding agent for establishing an adhesive bond between the junction surface as the gasket thermoplastic material is cooled to solidify the gasket and affix it to the junction surface. The mold piece is then removed away from the junction surface, and the part, with gasket affixed in place is ready for connection with the other part.
The method may also include coating the walls of the wall cavity other than the junction surface with a slip coat material after applying the release agent and before closing the mold. The slip coat material acts to form a skin on the surfaces of the gasket being formed other than the surface adjacent the junction surface.
In the practice of the foregoing methods, one part may be formed of polypropylene with the bonding agent comprising from about 5 to about 50 percent fatty acid dimer based on polyamide and the balance comprising isotactic and atactic polypropylenes in a ratio between about 1:1 and about 1:10, with bonding agent comprising less than about 30 percent of the gasket forming thermoplastic material.
Furthermore, the one part may be formed of nylon and the bonding agent may be an acrylic material comprising less than about 30 percent of the gasket forming thermoplastic material.
Also, the foregoing methods may be practiced with one part formed of polyethylene and the bonding agent being a hydrocarbon resin comprising less than about 30 percent of the gasket forming thermoplastic material.
Silicone is the preferred release agent in the practice of the foregoing method, and a slip coat material is a water-based polyurethane.
In a preferred form of the invention an elastomer composition adapted for use in forming gaskets affixed to one of two parts joined together in a gasketed joint includes at least one thermoplastic rubber material and a bonding agent capable of bonding the composition to the material from which the one part is formed upon solidification of the composition from the melt. An elastomer composition especially suited for use in forming gaskets affixed to polypropylene parts is one in which the bonding agent comprises from about 5 to about 50 percent fatty acid dimer based on polyamide and the balance comprise isotactic and atactic polypropylenes in a ratio between about 1:1 and about 1:10, the bonding agent comprising less than about 30 percent of said composition. An elastomer composition especially suited for use in forming gaskets affixed to nylon parts is one in which said bonding agent is an acrylic material comprising less than about 30 percent of said composition. Furthermore, an elastomer composition especially suited for use in forming gaskets affixed to polyethylene parts is one in which the bonding agent is a hydrocarbon resin comprising less than about 30 percent of the composition.